As disclosed in March, Advanced Organic Chemistry, Second Edition, McGraw-Hill, New York, 1977, pp. 501-502; Olah, Friedel-Crafts and Related Reactions, Volume 2, Interscience Publishers, New York, 1963-1964, pp. 659-784; U.S. Pat. No. 2,516,971 (Galigzenstein et al.); Canadian Patent 1,135,268 (Harris); and the references cited therein, it is known that aromatic compounds can be haloalkylated by reacting them with a hydrogen halide and an appropriate aldehyde, or with an .alpha.-halo-alkyl ether or an .alpha.-haloalkyl alkyl ether, in the presence of a Lewis acid or a proton acid as a catalyst, most commonly in the presence of zinc chloride.
The haloalkylations utilizing formaldehyde or a formaldehyde-derived ether have been successfully employed in providing fairly high yields of 1-halo-1-arylalkanes. Reasonably high yields of 1-halo-1-arylalkanes have sometimes also been obtained from haloalkylations utilizing higher aldehydes or ethers derived from them. However, it has frequently not been found possible to provide commercially acceptable yields of 1-halo-1-arylalkanes from higher aldehydes and ethers, especially when the aromatic compound has been one of the less reactive ones, such as a monoalkylaromatic hydrocarbon. There has been too much co-formation of diarylalkane by-product.
It would be desirable to find a way of increasing the 1-halo-1-arylalkane yields obtainable from such processes to provide a more economical method of preparing, the 1-halo-1-(4-alkyl-phenyl)alkanes used in known processes, such as those of U.S. Pat. No. 4,536,595 (Gardano et al.), Canadian Patent 1,197,254 (Francalanci et al.), British Patent 1,560,082 (Dynamit Nobel), Czechoslovakian Certificate of Authorship 219,752 (Palecek et al.), and Japanese Kokai 47-39050 (Miyatake et al.) and 51-111536 (Tokutake).
Definitions
As used herein, alkyl means straight or branched chain alkyl having 1 to 20 carbon atoms and includes, for example, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, secondary butyl, tertiary butyl, pentyl, isopentyl, neopentyl, hexyl, heptyl, octyl, 1-ethylhexyl, 1,1,3,3-tetramethylbutyl, nonyl decyl, dodecyl, tetradecyl, hexadecyl, octadecyl and eicosyl;
substituted phenyl and substituted naphthyl means phenyl or naphthyl substituted by at least one substituent selected from the group consisting of halogen (chlorine, bromine, fluorine or iodine), amino, nitro, hydroxy, alkyl, alkoxy which means straight or branched chain alkoxy having 1 to 10 carbon atoms, and includes, for example, methoxy, ethoxy, propoxy, isopropoxy, butoxy, isobutoxy, secondary butoxy, tertiary butoxy, pentyloxy, isopentyloxy, hexyloxy, heptyloxy, octyloxy, nonyloxy and decyloxy, haloalkyl which means straight or alkyl having 1 to 8 carbon atoms which is substituted by at least one halogen, and includes, for example, chloromethyl, bromomethyl, fluoromethyl, iodomethyl, 2-chloroethyl, 2-bromoethyl, 2-fluoroethyl, 3-chloropropyl, 3-bromopropyl, 3-fluoropropyl, 4-chlorobutyl, 4-fluorobutyl, dichloromethyl, dibromomethyl, difluoromethyl, diiodomethyl, 2,2-dichloroethyl, 2,2-dibromoethyl, 2,2-difluoroethyl, 3,3-dichloropropyl, 3,3-difluoropropyl, 4,4-dichlorobutyl, 4,4-difluorobutyl, trichloromethyl, trifluoro-methyl, 2,2,2-trifluoroethyl, 2,3,3-trifluoropropyl, 1,1,2,2-tetrafluoroethyl and 2,2,3,3-tetrafluoropropyl; PA1 phenylalkyl means that the alkyl moiety is straight or branched chain alkyl having 1 to 8 carbon atoms and includes, for example, benzyl, 2-phenylethyl, 1-phenylethyl, 3-phenylpropyl, 4-phenylbutyl, 5-phenylpentyl, 6-phenylhexyl and 8-phenyloctyl; and PA1 substituted phenylalkyl means the above-mentioned phenylalkyl which is substituted by at least one substituent selected from the group consisting of halogen, amino, nitro, hydroxy, alkyl, alkoxy and haloalkyl on the phenyl nucleus.
The Invention
It has now been found that 1-chloro-1-arylalkanes can be continuously prepared with minimum co-formation of the diarylalkane by-product, even when the aromatic reactant is a monoalkyl-aromatic hydrocarbon, by adding one molar proportion of an aromatic compound having at least one free ring position to from about 0.1 to about 1.5 mol of an aldehyde with agitation at a temperature in the range of about -35.degree. C. to about 0.degree. C. and in the presence of at least one molar proportion of hydrogen chloride and about 2-15 molar proportions of hydrogen sulfate.
The addition of the above-named components form a reaction mixture from which is removed a reaction effluent stream. This stream is comprised of unreacted starting materials (typically about 40% to about 60% of the starting material is not converted), the desired aryl-substituted ethyl halide (also termed herein as a 1-halo-1-arylalkane) and higher molecular weight by-products such as dimers, trimers and the like. Typically, the by-product produced in greatest yield is the dimer, e.g., where benzene is used as the aromatic compound and acetaldehyde as the aldehyde in the presence of hydrogen chloride and hydrogen sulfate, the dimer is 1,1-diphenylethane.
It has been discovered that if the rate of addition of all of the components is adjusted to provide a mixture of the above-named components in the ranges indicated (and the removal of a quantity of the reaction mixture is held substantially equal to the rate of addition of the reactants), then the amount of desired chloralkylated product is maximized while minimizing the quantity of higher molecular weight by-products--typically dimer. As such, when mixtures of the components are employed outside the ranges indicated, the yield of chloroalkylated product diminishes and the ratio of chloroalkylated product to dimer is seen to decrease. Within the ranges noted, the yields and ratios reach a substantially constant value. Thus, preferred ranges are about 0.1 to 1.5 mol of acetaldehyde per mol of the aromatic compound. Outside of the range and preferred range disclosed, variability of yield and ratio during the course of reaction effluent stream removal occurs.
Further, if the continuous removal of reaction effluent stream does not occur, i.e., a batch reaction, yields of product and ratio of product to by-product decrease dramatically.
The aromatic compound employed in the practice of the invention may be a carbocyclic aromatic compound, e.g., an unsubstituted aromatic hydrocarbon, such as benzene, naphthalene, anthracene, phenanthrene, etc.; a polyalkylaromatic hydrocarbon such as xylene, pseudo-cumene, mesitylene, etc.; and aromatic hydrocarbon bearing a substituent such as halo, cyano, nitro, hydroxy, alkoxy, phenoxy, alkylthio, etc. (e.g., the 2-, 3-, and 4-chloronitrobenzenes, the 2-, 3-, and 4-fluoronitrobenzenes, 4-chloronitrobiphenyl, 6-methoxynaphthalene, phenoxybenzene, etc.); or it may be a heterocyclic aromatic compound, such as a chlorocarbazole, 2-phenyl-1-isoindolinone, 6-fluoro-5-nitroquinoline, etc. However, because of the commercial interest in their haloalkylated products and the difficulty that has previously been encountered in preparing the desired 1-halo-1-arylalkanes, the preferred aromatic compounds are monoalkylaromatic hydrocarbons, such as substituted phenyl or substituted naphthyl illustrated by 1-methylnaphthalene, 2-methylnaphthalene, 2-methoxynaphthalene, and the various monoalkylbenzenes, e.g., the methyl-, ethyl, propyl-, isobutyl-, sec-butyl-, t-butyl-, isopentyl-, t-pentyl-, and hexylbenzenes. The most preferred aromatic compounds are the monoalkylbenzenes wherein the alkyl group contains 1-5 carbons.
The aldehydes of use herein have the formula ##STR1## where R.sub.1, R.sub.2 and R.sub.3 are the same or different and are hydrogen, alkyl, phenylalkyl or substituted phenylalkyl. Preferably, R.sub.1 is alkyl having 1 to 10 linear or branched carbon atoms and R.sub.2 and R.sub.3 are the same as R.sub.1 or are hydrogen. Most preferably, R.sub.1 has 1 to 6 carbon atoms and R.sub.2 and R.sub.3 are hydrogen. Particularly preferred is where R.sub.1 is alkyl of 1 to 3 carbon atoms. Acetaldehyde is a very useful reactant in the process of the present invention.
The amount of aldehyde employed in the chloroalkylation reaction may be the stoichiometric amount, i.e., the amount which provides one R.sub.1 group per molecule of aromatic hydrocarbon. In some cases, less than this amount may be employed. However, it is generally preferred to employ an amount that provides at least one R.sub.1 group per molecule of aromatic compounds. Most preferred is about one mole of such aldehyde per mole of haloalkylated product. There does not appear to be any maximum to the amount of aldehyde that may be used other than the maximum that economics permit.
As in known processes, the chloroalkylation is conducted in the presence of an acid catalyst, preferably hydrogen sulfate. In order to avoid the presence of too much water in the reaction mixture, as well as to take advantage of commercially-available materials, the hydrogen sulfate is generally introduced in the form of 88-98% sulfuric acid. The amount employed is generally such as to provide at least about one mol, preferably at least about 2-6 moles, per mol of aromatic compound; and it ordinarily should not exceed about 15 moles per mol of aromatic compound. It should be noted that oleum may be used and directly added to the reaction mixture. It combines with the water produced in the reaction to yield sulfuric acid at the desired concentration.
The amount of hydrogen chloride used in the reaction is usually at least about one equivalent, based on the amount of aromatic compound; and it is generally introduced by bubbling it through the reaction mixture or by pressurizing the reaction vessel with it.
Since improved yields of 1-chloro-1-arylalkane are not obtained without it, the use of the hydrogen chloride is critical.
The reaction is usually conducted at a reaction temperature in the range of about -35.degree. C. to about 0.degree. C., preferably about -35.degree. C. to about -15.degree. C., most preferably about -30.degree. C. to about -20.degree. C., in order to achieve the maximum advantages of the invention. The higher temperatures generally favor higher conversions, while the lower temperatures are apt to favor higher chloroalkylation product/diarylalkane ratios.
The manner of combining the ingredients does appear to be somewhat important. For example, (1) the aldehyde may be dissolved in the aromatic compound and added to the catalyst while bubbling hydrogen chloride through the reaction mixture, or (2) the pure or crude aldehyde, the aromatic compound, and the catalyst may be combined in either fashion in a reaction vessel which is pressurized with the hydrogen chloride, etc. However, the best addition method is to add all reactants to a well-mixed stream of reaction mixture.
The invention is useful as an alternative method of preparing 1-chloro-1-arylalkanes from aromatic compounds that are known to be capable of providing high yields of such products by known chloroalkylation techniques. However, it is particularly advantageous as a method of preparing 1-chloro-1-arylalkanes from the less reactive aromatic hydrocarbons, such as monoalkyl-benzenes, that have not previously been found to be capable of providing high yields of such products by chloroalkylation processes other than chloromethylations.
It should be noted that the process of the present invention is most preferably operated in a continuous, stirred reaction. Disadvantageously, when the process is performed in a continuous plug flow reaction rather than observing improved yields of haloalkylated product, depressed yields are obtained, closely paralleling semi-batch systems.
As is known, the products obtained by the process are useful as internal standards, intermediates for the preparation of monomers, detergents, pharmaceuticals, etc. When they are used as chemical intermediates, they may be subjected to the same reactions as have previously been used to convert them to desired products. For example, the 1-chloro-1-phenylethanes can be dehydrohalogenated in any known manner to provide styrenes which can then be polymerized by known techniques.
A particularly interesting application of the 1-chloro-1-(4-alkylphenyl)ethanes which are prepared in a preferred embodiment of the invention is as intermediates for the preparation of ibuprofen and related pharmaceuticals. When they are used in such applications, they may be converted to the desired products in any suitable manner. For example, they may be reacted with carbon monoxide in the presence of a carbonylation catalyst and then acidified to the corresponding propionic acids as in Gardano et al., Francalanci et al., or Dynamit Nobel; or they may be cyanated and then acidified to the corresponding propionic acids as in Palecek et al. or Tokutake. Another useful synthesis involves reacting the compounds with magnesium, carbonating the resultant Grignard reagents with carbon dioxide, and acidifying the carbonated product to the propionic acid as in Miyatake et al.